Reactions of organic compounds always involve the making and breaking of covalent bonds. A covalent bond may break in two fundamentally different ways.
When a bond breaks such that one fragment takes away both electrons of the bond, leaving the other fragment with an empty orbital, this kind of cleavage is called heterolysis (Greek: hetero, different, + lysis, loosening or cleavage). Heterolysis produces charged fragments or ions and is termed an ionic reaction. The broken bond is said to have cleaved heterolytically:
Heterolysis of a bond normally requires that the bond be polarized:
Polarization of a bond usually results from differing electronegativities of the atoms joined by the bond. The greater the difference in electronegativity, the greater is the polarization. In the instance just given, atom B is more electronegative than A.
Even with a highly polarized bond, heterolysis rarely occurs without assistance. The reason: Heterolysis requires separation of oppositely charged ions. Because oppositely charged ions attract each other, their separation requires considerable energy. Often, heterolysis is assisted by a molecule with an unshared pair that can form a bond to one of the atoms:
Formation of the new bond furnishes some of the energy required for the heterolysis.
When a bond breaks so that each fragment takes away one of the electrons of the bond, this process is called homolysis (Greek: homo, the same, + lysis). Homolysis produces fragments with unpaired electrons called radicals.